It is well known that the visible reflection of light rays from the surface of a planar substrate may be reduced by the deposition thereon of various singular or multiple layer coatings. In the optics and vision glazings fields, the primary emphasis has been directed toward reducing the observable coloration of the glazing as well as the reflection of light in the visible spectrum in the range from 4,000 to 7,000 Angstroms. Manufacturers and purchasers of anti-reflection glazings are particularly concerned with the color purity of such products, realizing that a lower color purity glazing appears more color-neutral than a high color purity product. Therefore, low color purity glazings are much more desirable for use as automotive and architectural windows.
Single layer anti-reflection coatings are well known. However, since the visible spectrum extends over a relatively wide wavelength band, and a single anti-reflection layer is principally designed to nullify reflection at a single wavelength, its use does not provided satisfactory results over the entire visual region. This limitation is particularly critical for optical elements and vision glazings made from transparent materials having refractive indices in the range from about 1.45 to about 1.9.
Three-layer anti-reflection coatings have greatly improved optical and vision characteristics over the known single layer coatings. Generally, the third layer which is the outer layer exposed to the atmosphere is designed to minimize the reflectance, and has a low refractive index with an optical thickness of one-quarter wavelength. As is well known to those skilled in the art, the optical thickness is the physical thickness multiplied by the index of refraction of the material. The optical thickness is normally designated as a fraction of the wavelength of the design light ray passing through the coating. Generally speaking, the design wavelength useful for designing anti-reflection coatings for optical elements and vision glazings is about 5,500 Angstroms.
In the conventional three-layer anti-reflection coatings known in the prior art, the second or middle layer has a high refractive index and an optical thickness of one-half wavelength. A one-half wavelength optical thickness coating does not alter the optical characteristics of the other layers, and therefore has no effect on the residual reflectance. However, it will augment the anti-reflection effect of the multiple layer coating for light rays having a range of wavelengths on either side of the design wavelength.
The first layer of the three layer anti-reflection coatings of the prior art, positioned between the middle layer and the substrate, generally has a medium refractive index and an optical thickness of one-quarter of a design wavelength. Generally, this layer acts as a matching layer between the other two layers and the substrate.
The individual anti-reflection layers generally comprise dielectric materials which are deposited as discrete laminae by conventional methods such as, for example, sol-gel coating, sputtering, or chemical vapor deposition. It is also known to employ two different dielectric materials having different refractive indicies which are co-deposited to achieve an arithmetic mean refractive index, or a continuously varying refractive index normal to the coated surface. The continuously varying refractive index may be achieved by varying the deposition rate of the two dielectrics with respect to the thickness of the deposited layer. Optical interference caused by these multiple layer dielectric materials generally produces transmitted and reflected colors which vary in color purity or intensity, frequently making the coated glazing unusable as an automotive or architectural window.
U.S. Pat. No. 4,771,167 to Boulos et al. discloses a three layer anti-reflection coating which is deposited on one of the inner surfaces of a laminated electrically heatable vehicle windshield. The coating reduces the reflectance and increases the visible transmittance of the windshield, to enable the electrically heatable windshield to meet U.S. federal regulations requiring an Illuminant A visible light transmittance greater than 70%. The disclosed layers which are sequentially deposited onto the glass substrate are, consecutively: SiO.sub.2 -TiO.sub.2, 787 Angstroms thick; TiO.sub.2, 635 Angstroms thick; and SiO.sub.2, 934 Angstroms thick. This coating, however, is not designed to minimize the total reflectance of the coated and uncoated surfaces of the windshield when installed at a high angle of incidence while at the same time reducing the color purity or intensity of the perceived coloration of the coated windshield.
U.S. Pat. No. 2,478,385 to Gaiser discloses a three layer anti-reflection film for glass surfaces based upon the aforementioned 1/4-1/2-1/4 wavelength theory. The disclosed layers comprise: SnO.sub.2, 1,400 Angstroms; TiO.sub.2, 2,800 Angstroms; and SiO.sub.2, 1,400 Angstroms. Likewise, the anti-reflection layers disclosed in U.S. Pat. No. 3,185,020 to Thelen are based upon the 1/4-1/2-1/4 wavelength theory, and comprise a third layer of MgF.sub.2 (refractive index=1.38), a second layer of ZrO.sub.2 (refractive index=1.9-2.3), and a first layer adjacent the glass surface of CeF.sub.2 (refractive index=1.8-1.85).
U.S. Pat. No. 3,934,961 to Itoh et al. discloses a three-layer anti-reflection film based upon the 1/4-1/2-1/4 wavelength theory, wherein the third and second layers consist of conventional dielectrics and the first layer adjacent the glass surface consists of Al.sub.2 O.sub.3 -ZrO.sub.2.
The aforementioned prior art, in which the three-layer anti-reflection coatings are based upon the 1/4-1/2-1/4 wavelength theory, is directed toward reducing or eliminating the reflection from only that surface of the substrate to which the coating is adhered. The prior art does not teach nor suggest three-layer anti-reflection coatings which would be useful for reducing the reflection from both the coated and uncoated surfaces of a transparent sheet, or the multiple coated and uncoated surfaces of laminated transparent sheets (which sheet or sheets are oriented at high angles of incidence), while at the same time reducing the color purity to an almost visually undetectable level. The 1/4-1/2-1/4 wavelength theory of optics is applicable only to light reflected from a single surface, and cannot predict the color purity of a multi-layered structure. In fact, it is well-known that most multi-layered structures display intense iridescence characterized by very high color purity values.
Other three-layer anti-reflection coatings are known which do not conform to the 1/4-1/2-1/4 wavelength theory, but which are effective in reducing reflected light from single surfaces. Such coatings are generally produced by experimenting with various combinations and permutations of dielectric materials, refractive indices, and thicknesses, and cannot be derived by resorting to any particular theory of optics. One such coating is disclosed in U.S. Pat. No. 3,712,711 to Adachi, wherein the first layer is a material of 0.3 wavelength thickness and a refractive index of 1.36, the second layer is a material of 1/30 wavelength thickness having a refractive index of 2.4-2.7, and the third layer is a material of 0.4 wavelength thickness and a refractive index of 1.65-1.75. As in the previously cited prior art, however, this coating is designed to reduce the reflection only from a single surface of the substrate oriented at a zero angle of incidence, and is not designed to minimize the color purity of the resultant article. The recited materials of construction, thicknesses, and refractive indices would not be useful for reducing the reflection of light from both the coated and uncoated surfaces of a transparent sheet oriented at a high angle of incidence while at the same time minimizing the color purity thereof.
Several publications recite mathematical formulae for determining the refractive indicies and thicknesses required for the individual laminae of three-layer, anti-reflection coatings useful for reducing the reflection of light rays from a singular coated surface of a planar substrate. See for example Thetford, A., "A Method of Designing Three-Layer Anti-Reflection Coatings," Optica Acta, v. 16, n. 1 (1969) 37-43. Other publications such as, for example, Turbadar, T., "Equi-Reflectance Contours of Triple-Layer Anti-Reflection Coatings," Applied Optics Section, Imperial College, London (1964) 195-205, disclose similar formulae, including factors for determining the path lengths of light traveling through the anti-reflection coating laminae, which at high angles of incidence are naturally greater than the path lengths of light rays which would otherwise travel normal to the coated surface. However, Turbadar only discloses actual laminae thicknesses which are derived from the 1/4-1/2-1/4 wave theory. Furthermore, Thetford and Turbadar do not disclose three-layer anti-reflection coatings which account for the reflectance contributions of surfaces other than the one to which the coating is adhered, such as would be encountered in a vision glazing having a first coated surface and a second coplanar uncoated surface. Finally, the two articles do not disclose coated structures having minimized color purity values.
It would be desirable to deposit a three-layer, anti-reflection coating on a surface of a transparent glazing, which coating would minimize the total reflection of visible light from the coated and uncoated surfaces when the transparent glazing is oriented at a high angle of incidence with respect to a source of the visible light while at the same time minimizing the color purity thereof to a level acceptable for use as an automotive or architectural glazing.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention, nor that the cited prior art when considered in combination suggests the present invention absent the teachings herein.